ok here it goes..  you must have understood the theory by now. let me explain with a scenario. you are connected to the internet. and for that you require to have an ip with your isp. now an attacker can find out your ip and it requires not much expertise. and now what he can do for a spoof is inject your ip into the packets and send to a server or some location for denial of service. and he keeps on sending it and sending it. and he can have the sender as your address. now the response from the server comes to you and thus he is safe from too many packets running to him. thus the server gets overwhelmed and so does you. This is an overall idea about what I guess the technique is. Correct me if I got it wrong.

Note: Above information only for educational purposes.It may or may not be authentic.